3o6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



virtue of the differential rates of depolarization a 

 neuron is able to fire repetitively, resembling the 

 oscillatory discharge and recharge of two thyratron 

 tubes with two different-valued condensers in the 

 circuit. This concept of Burns is apparently derived 

 from his observation that after-discharge of skeletal 

 muscle fibers may be caused by treatment with 

 decamethonium iodide, which is believed to pre\ent 

 the end plate membrane from repolarizing as rapidly 

 as the neighboring membrane of the muscle fiber. 



Local AJter-Discharges Involving Intrinsic 

 Neuronal Circuits 



Although individual neurons are capable of dis- 

 charging repetitively in response to a single stimulus, 

 long lasting activities in the central nervous system are 

 mostly manifestations of neuronal discharges resulting 

 from a self-re-exciting mechanism involving numerous 

 neurons arranged in closed circuits in the same struc- 

 ture. 



As demonstrated by Burns (12), an isolated slab of 

 cerebral cortex is able to discharge following a single 

 shock applied to the cortical surface. Such discharges 

 may last for many minutes or even hours in some 

 instances and are detectable not only in the circum- 

 scribed area directly under stimulation but also in 

 regions at some distance from it. They are evidently 

 not the repetitive firings of the directly stimulated 

 neurons but the responses of neurons synaptically 

 excited through neuronal circuits. After-discharges of 

 this kind usually develop increasing intensity and 

 then suddenly stop altogether at the climax. They 

 may resume the activity after a brief pause. In such a 

 case, the activity of individual neurons apparently 

 depends on the arrival of impulses from some other 

 neurons in the circuit for re-excitation. To perpetuate 

 the activity, the circulating impulses must be main- 

 tained above the liminal strength and arrive at the 

 next neuron at an opportune moment when the 

 excitability of the neuron is favorable. The abrupt 

 cessation or suspension of the after-discharge at its 

 climax is probably due to the postexcitatory depres- 

 sion of some neurons in the circuit which fail to 

 respond to the arriving impulses so that the circuit is 

 broken. The self-re-exciting circuits are present in 

 every part of the central nervous system where inter- 

 nuncial neurons exist. Many neurons, especially 

 those whose axons are short but have numerous 

 collaterals, constitute the main source for the elabora- 

 tion of local after-discharges. 



The acti\-ation of the internuncial neurons through 



collaterals has been demonstrated in the cerebral 

 cortex. The action potential of the motor cortex 

 produced by a single shock stimulation of the medul- 

 lary pyramid consists of the initial deflections with 

 short latency and in addition a rather prolonged 

 discharge which has a latency of 14 to 16 msec. It 

 cannot be interpreted as antidromic activity of the 

 directh' stimulated large p\ramidal neurons. Such 

 later components of the antidromic cortical potential 

 are variable, labile and more susceptible to the action 

 of anoxia, specific drugs (strychnine for instance) and 

 tetanic stimulation — showing characteristics of the 

 responses invoking synapses. When two successive 

 stimuli are applied to the medullary pyramid at short 

 intervals, the cortical response to the second is usually 

 blocked. The temporal course of the recovery process 

 is similar to that of orthodromically evoked potentials. 

 Unit activity of the internuncial neurons participating 

 in the development of such activity can be recorded 

 from different strata of the cortex with a micro- 

 electrode. Perhaps the most interesting is the fact that 

 the large pyramidal neurons whose axons ha\e been 

 stimulated originally can be re-excited synaptically by 

 their own collaterals. The discharge of the same 

 pvramidal neuron resulting from the internuncial 

 activity is detectable from the point on the medullary 

 pvramid where the single shock stimulus has been 

 first applied. Thus, the action of the self-re-exciting 

 circuit is completed. The particular significance of the 

 collateral acti\ity of the pyramidal fibers lies in the 

 fact that they constitute a part of the feed-l^ack 

 mechanism in the cerebral cortex by which a message 

 is sent back to the original dispatcher for modification 

 of the subsequent responses. If the feed-back im- 

 pulses are sufficiently strong and arrive when the 

 brain excitability is in the most favorable condition, 

 it is even possible to initiate a rhythmic after-dis- 

 charge of the efferent neurons. It is believed that the 

 prolonged epileptiform after-discharges following 

 strong stimulation of the motor cortex and the seizures 

 in pathological cases are produced, at least in part, by 

 a feed-back mechanism through the collaterals in the 

 closed chains ot neurons. 



Rhythmic After-Discharges Involving Long Neuronal 

 Circuits: Corticothalamic Reverheratory Activity 



The primary response of the sensory cortex to an 

 afferent volley is often followed by a train of regularly 

 spaced surface-positive waves with intervals ranging 

 from 50 to 150 msec. (14). The frequency of the 

 repetitive waves seems to l)e independent of the stimu- 



